The present invention relates to the changing of frequencies with suppression of the component at the frequency of the local oscillator that has enabled the changing.
Frequency changing is achieved by mixing between an input signal at a frequency that is generally called an intermediate frequency and a signal called a local frequency signal given by a local oscillator. This changing gives a first component which is the useful signal, a second component at the local frequency and a third component which is an image of the first component with respect to the second component. The word "image" herein implies that the differences between the local frequency and the frequencies of the first and third components have the same value in absolute terms but have opposite signs.
Given that the first component which is the useful component has to be obtained alone, there are known ways of greatly attenuating the other two components by means of a filter placed at the output of the mixer that has carried out the changing. This filter is a passband filter centered on the frequency of the useful signal which shall be called a useful frequency. This filter attenuates the signals at the local frequency and at the frequency of the image component which shall be called the image frequency. The suppression of the local frequency is the result that is the most difficult to obtain for, of the two frequencies to be suppressed, it is this frequency that is the closest to the useful frequency.